In Chicago, principals were given the ability to dismiss the probationary teachers—those with five years of experience or less—without completing elaborate documentation or attending a dismissal hearing, under a 2004 collective bargaining agreement between the 409,000-student school district and the Chicago Teachers Union.

In return for the flexibility, the district expanded the pool of teachers who were placed on a tenure track. The policy change went into effect for the 2004-05 school year.

The study [Prof. Brian Jacob of U. of Michigan] examines the effects of the policy from that year through the 2006-07 school year, and compares teacher-absence rates from before and after the policy was implemented for probationary vs. tenured teachers. Mr. Jacob used payroll records to review the teacher-absence data, and the academic years 2001-02 through 2003-04 were used as the pre-policy period for comparison purposes.

In the two years before the policy change, the study found, the average teacher was absent about eight times a year, a figure that declined starting in 2005, especially among nontenured teachers. By 2007, that number for probationary teachers was just above six times a year.